Polymers that contain ethylenic unsaturation can be hydrogenated to improve properties, and in particular to improve stability. Such hydrogenation processes are disclosed in, for example, U.S. Pat. Nos. 3,700,633 and 3,700,748, the disclosures of which are incorporated herein by reference. These processes include combining a solution or suspension of the polymer with Group VIII metal alkoxide or carboxylate and a metal alkyl. Group VIII metals in general will function as the active metal in these systems, and in particular, nickel, iron, cobalt and palladium are known to be effective. This mixture is then contacted with hydrogen at an elevated pressure and temperature, resulting in hydrogenation of ethylenic unsaturation.
A particularly difficult aspect of these hydrogenation processes is removal of the hydrogenation catalyst metals after hydrogenation is complete. After hydrogenation is complete, the metal components are typically removed from the polymer solution by contacting the polymer solution with an aqueous phase of, for example, carboxylic acid, sulfuric acid or phosphoric acid. An oxidizing agent may also be used to react with the metal or metal compound either prior to or simultaneously with the carboxylic acid, sulfuric acid or phosphoric acid. The aqueous phase is then separated from the polymer solution.
Removal of the hydrogenation catalyst metal is important because small amounts of hydrogenation catalyst metal are detrimental to many polymer properties, including stability and color.
U.S. Pat. No. 4,992,529 discloses a method for separating metal contaminants from organic polymer solutions using a high molecular weight monocarboxylic acid in combination with an inorganic acid. Although this method can be effective for many polymer and metal combinations, it has been found to be less effective than what is desired for other combinations. In particular, when anionically polymerized polymers are initiated with lithium, and the lithium is present in the final polymer solution in relatively high concentrations, removal of both the lithium and nickel hydrogenation catalyst metal becomes particularly difficult, and relatively large amounts of the high molecular weight monocarboxylic acid is required.
Removal of metals from a hydrogenated polymer solution is affected by the amount of water remaining in the polymer cement after the metals removal step is complete. This is because the remaining metals are, after contact of the solution with an acidic aqueous solution, often concentrated in the small amount of water entrained within the polymer cement. Effective separation of the water from the polymer cement is necessary for the effective removal of metals.
It is therefore an object of the present invention to provide a method to hydrogenate polymers containing ethylenic unsaturation in a polymer cement wherein the hydrogenated polymer cement contains not more than about 0.8 percent by weight entrained water. In another aspect, it is an object to provide such a method wherein hydrogenation catalyst metals are effectively removed without addition of excessive amounts of a contaminant.